Creating polar diagrams in GRASS GIS

There are so many functions in GRASS GIS that it is easy to miss one or two. One functions I recently came across through this tutorial is the d.polar function.

This function draws a polar diagram of angle map such as aspect or flow directions. You can access it by typing d.polar on the command line in GRASS. Just fill in the required information (the input map) and name of the EPS output file (optional, you can also choose to have it displayed on screen).

Alternatively, fill in all on the command line, like in the example below. In the example, I first plot the distribution of the slope aspect in an area around Mount Kilimanjaro in Tanzania (Figure 1).

Vegetation map of an area around Mount Kilimanjaro (for more information about the vegetation map, see this link

d.polar map=aspect@PERMANENT eps=aspect_1.eps

This gives the polar diagram in Figure 2. As you can see, there is not a clear prevailing direction of the slope aspect. The strong spikes in NW, SW, SE and NE directions indicate an over representation of those angles, which might be because the DEM used to calculate the slope aspect map is an integer map.

Figure 2: Polar diagram of the slope aspect distribution of the areas shown in Figure 1.

Next, I plot the slope aspect distribution of the Afromontane dry transitional forest in the area (the dark green area in Figure 1). To do this, I mask out all other areas and run d.polar again.

r.mask input=pnv_vecea2_agg@ConsStat maskcats=10
d.polar map=aspect@PERMANENT eps=aspect_2.eps

As you can see in Figure 3, this simple exploratory analysis shows that this type of forest occurs more often on the southern slopes (I know, I could have told as much directly from figure 1).

Polar diagram of the slope aspect distribution in the areas mapped as Afromontane dry transitional forest in Figure 1

This becomes even clearer when combining the two figures in Figure 4. I did not find an option to create this combined polar diagram directly, so I used GIMP to combine the two diagrams above.

Figure 4: In red the polar diagram of the slope aspect distribution of the whole area in Figure 1. In purple the polar diagram of the areas mapped as Afromontane dry transitional forest within the same area.

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About pvanb

I am a tropical forest ecologist with a focus on spatial and temporal patterns and processes at population and ecosystem level. I am furthermore very interested in issues related to conservation and sustainable use of biodiversity and natural resources under current and future climates. I have worked in the Middle East (Syria and Lebanon) and South America (Brazil) and in Eastern Africa (Kenya).
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4 Responses to Creating polar diagrams in GRASS GIS

  1. Pingback: Creating polar diagrams with confidence intervals in R | Ecostudies

  2. Pingback: l'aménagerie » Blog Archive » Ce que j’ai lu cette semaine

  3. swapan ghsoh says:

    Is this works on windows system

    • pvanb says:

      I haven’t tried it, but assume it does. Just try it out, type d.polar on the command line in GIS. It should pop up the function window

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